Gearless turntables are available at the present time in which the angular position of the table is set by matching a line mark on the turntable with a line mark on a stationary base. The precision capable of being achieved depends on the eyesight of the individual and on the amount of illumination available. At best, it is difficult to adjust the position of the table to within .+-.0.004 inch of the desired precise angular setting.
Line marks may be used for rough machining, but are unsatisfactory where greater accuracy is needed. To achieve greater accuracy, as for example where the tolerance is .+-.0.004 inch, a sine bar and indicator have been used. However, the workpiece to be machined does not always have a convenient surface or points to attach the sine bar for indication. In that event, another parallel bar or angle plate has to be used in combination with the sine bar for indication purposes. Then, it is necessary to run the indicator back and forth until it shows a zero reading all along the indicated area.
There are also available at the present time geared rotary tables that provide sufficiently precise set-ups for all angular positions. However, it requires ninety turns of the crank to rotate them a full revolution. Some have a direct indexing arrangement for only every 10.degree.. Ninety crank turns is too much cranking, and moreover the round tables provide space for only a limited range of rectangular details.
The apparatus of my invention eliminates the need for visual judgment to match up line marks, while at the same time providing a high degree of accuracy and repeatability. The total error is limited to .+-.0.001 of an inch. Considering that the same procedure is used for setting up all the desired angular positions, the error will be the same on all settings, thus minimizing itself, and accordingly the machined surfaces will be accurate in relation to each other.
In the apparatus about to be described, 360 locating holes are spaced at one degree intervals about the periphery of the turntable. Two indexing pins are provided which may be inserted in any of the holes. Indexing blocks are also provided having prongs provided with abutement surfaces for contact with the pins. One of the blocks employed has a single prong provided with a micrometer screw having the abutment surface thereon. Another of the blocks has two spaced prongs between which a pin is adapted to extend.
A turntable normally rests in friction contact on two pads during a machining operation. The table may be lifted from the pads by a manually operable lifting machanism in order to be rotated to another position. Anti-friction ball bearings support the table on the lifting mechanism when it is raised, to facilitate rotation.